For months the former Massachusetts governor has shown little willingness to assail his Republican competitors, staying above the fray and focusing all of his criticism on President Barack H. Obama. But in one night, Romney became the most prominent aggressor in a growing effort by the GOP field to derail front-runner Rick Perry. And in doing so, Romney may have started to ease concern within the GOP establishment over the strength of his candidacy.

“Those doubts were erased,” Steve Duprey, a Republican National Committee member from New Hampshire who is unaligned in the race, said Tuesday, a day after a Republican presidential debate in Tampa, Fla., where Romney tested out his on-the-attack approach. “In a dominant fashion, he showed he could take and fend off Governor Perry’s blows and at the same time deliver sound, hard-hitting policy criticisms of Governor Perry. After that debate, Romney looked strong. Perry looked dazed.”

It’s the beginning of a new phase in Romney’s campaign; aides say the time has come to contrast Romney’s record with those of his opponents and they expect Romney will keep the pressure on Perry, specifically. Criticism on issues like immigration, Social Security and jobs is expected on the campaign trail, in an upcoming debate next week, and, perhaps, in TV and radio ads eventually.

The shift reflects a growing sense of urgency in Romney’s campaign and others that Perry must be knocked down before he becomes too strong and runs away with the nomination.

For months, Romney sat atop public opinion polls across the nation and in early primary states, seemingly the top preference of Republican voters who denied him their party’s presidential nomination in 2008. He spent much of the year ignoring darts from his rivals.

Then Perry entered the race a month ago and immediately shot to the top of polls. And, within days, Romney started drawing contrasts with Perry without naming him, highlighting his own business background while generally noting that there also were “career politicians” in the race. It was an obvious reference to Perry, who – like some other candidates – has spent most of his adult life in and around politics. Even when asked pointed questions, Romney was careful not to directly engage.

He showed signs of a willingness to go after Perry in their first debate together last week, but he shifted his strategy completely during the opening minutes of Monday’s debate as the candidates sparred over Social Security, the program Perry has repeatedly described as a “Ponzi scheme” that may violate constitutional principles.

Romney quickly became Perry’s lead interrogator.

“The question is, do you still believe that Social Security should be ended as a federal program as you did six months ago when your book came out and returned to the states or do you want to retreat from that?” Romney asked Perry.

“I think we ought to have a conversation,” Perry said before being cut off.

“We’re having that right now, governor. We’re running for president,” Romney quipped, forcing Perry to defend a controversial position about the popular retirement program in a state with a huge retired population.

“Governor, the term ‘Ponzi scheme’ is what scared seniors, Number One. And Number Two, suggesting that Social Security should no longer be a federal program and returned to the states and unconstitutional is likewise frightening,” Romney said.

It’s unclear whether Romney’s shift will resonate with voters.

Perry has led in several recent national polls, most recently earning 30 percent to Romney’s 18 percent in a CNN survey taken from Sept. 9-11. National polling, however, is far less significant than voter sentiment in the early voting states like Iowa, New Hampshire, and South Carolina, which will help shape the race early next year. There is scant polling so far in those states.

Romney will get some help in trying to tear down Perry.

Romney may have already benefited from attacks by Minnesota Rep. Michele Bachmann and Texas Rep. Ron Paul, who have been strong Perry critics all along.

On Monday, they lashed out at Perry from the right, criticizing his positions on immigration, taxes and public health.

“Our taxes have doubled since he’s been in office. Our spending has gone up double. Our debt has gone up nearly triple,” Paul said of Perry’s decade as governor. “So I would put a little damper on this, but I don’t want to offend the governor, because he might raise my taxes or something.”

Bachmann went after Perry for trying to require vaccinations on 11- and 12-year old girls to prevent a sexually transmitted disease that causes cervical cancer.

“To have innocent little 12-year-old girls be forced to have a government injection through an executive order is just flat out wrong,” Bachmann said. “That should never be done. It’s a violation of a liberty interest.”

Perry defended his record the best he could, acknowledging a mistake by bypassing the state legislature to force the vaccines and highlighting job growth in Texas under his watch.

In the coming months, look to Romney to ensure Perry doesn’t get off easy.